1. Field of the Invention
The present invention relates generally to flow meters used to measure a flow rate of a fluid in a fluid system, and more particularly, to a flow meter capable of measuring a mass flow rate of a fluid, even if a single-phase or two-phase flow occurs in a vertical pipe or a stratified flow occurs in a horizontal pipe.
2. Description of the Related Art
FIG. 1 is a side sectional view of a pipe 1 equipped with a conventional pitot tube 10, with the pitot tube 10 comprising a double pipe (not shown). As shown in FIG. 1, the conventional pitot tube 10 is configured such that a total pressure hole 11 and a static pressure hole 13 are formed at predetermined positions in the double pipe. The pitot tube 10 is installed such that the total pressure hole 11 is opposite to the direction of fluid-stream movement. A total pressure acting on the total pressure hole 11 is a sum obtained by adding a static pressure to a dynamic pressure due to a fluid flowing in the pipe. Only the static pressure acts on the static pressure hole 13. Further, a pressure impulse line 15 extends from the double pipe having the total pressure hole 11 and the static pressure hole 13. As shown in FIG. 1, a differential pressure transmitter (not shown) connected to the pressure impulse line 15 measures the difference (ΔP) between a pressure acting on the total pressure hole 11 and a pressure acting on the static pressure hole 13. By the measured result, a flow velocity of the fluid is calculated. The flow velocity measured by the pitot tube 10 is a flow velocity at a position where the pitot tube 10 is located, but not an average flow velocity of the fluid flowing along the pipe 1. Thus, in order to find the average flow velocity of the fluid in the pipe 1, the flow velocity of the fluid must be measured at many positions several times while changing the insertion depth of the pitot tube 10, and then the mean flow velocity must be calculated.
FIG. 2 is a side sectional view to show a conventional average pitot tube 20, and FIG. 3 is a front sectional view of the conventional average pitot tube 20. In this case, the average pitot tube 20 comprises a double pipe (not shown). In the following description, those elements common to the pitot tube, of FIG. 1 and the pitot tube of FIGS. 2 and 3 will carry the same reference numerals.
As shown in FIGS. 2 and 3, a plurality of total pressure holes 21 and a plurality of static pressure holes 23 are formed on the double pipe of the conventional average pitot tube 20. A pressure impulse line 25 extends from the double pipe to be connected to a differential pressure transmitter (not shown). The principle of measuring the flow velocity of a fluid flowing in the pipe 1 remains the same as that of the pitot tube 10 of FIG. 1.
As described above, the pitot tube 10 is problematic in that the flow velocity of the fluid flowing in the pipe 1 must be repeatedly measured at many positions several times, so that it is complicated to measure the flow velocity and a longer time is required to measure the flow velocity. In order to solve the problems, the average pitot tube 20 has been proposed. The average pitot tube 20 is constructed so that total pressures are measured at a plurality of total pressure holes 21, and static pressures are measured at a plurality of static pressure holes 23. Thereby, the total pressures and the static pressures are simultaneously measured at several positions in the pipe 1, thus allowing an average flow rate of the fluid to be conveniently calculated.
FIGS. 4 and 5 show a local bidirectional flow tube 30. As shown in FIGS. 4 and 5, the local bidirectional flow tube 30 includes a cylindrical body 31, a partition plate 33, and a pressure impulse line 35. In the following description, those elements common to the pitot tubes of FIGS. 1 and 2 and the flow tube of FIGS. 4 and 5 will carry the same reference numerals.
The principle of measuring a flow velocity using the local bidirectional flow tube 30 is similar to the measuring principle using the pitot tube 10 of FIG. 1 and the average pitot tube 20 of FIG. 2. As for a fluid flowing forward, when the local bidirectional flow tube 30 is installed as shown in FIG. 4, the total pressure obtained by adding the static pressure to the dynamic pressure due to the fluid flowing in the pipe acts on the A part. On the other hand, a back pressure acts on the B part. At this time, the back pressure is slightly less than the static pressure due to a suction effect caused by a flow velocity of the fluid of the B part. The pressures acting on the A part and the B part are transmitted to the differential pressure transmitter through the pressure impulse line 35, so that the pressure difference between the A part and the B part is measured. Thereby, the average flow velocity of the fluid is calculated under any flow condition.
In the case of using the above-mentioned average pitot tube 20, when a two-phase flow may occur in the pipe 1, for example, when a liquid flows along the bottom of the pipe 1 and gas flows separately above the liquid, a pressure difference is generated between the total pressure holes 21, so that the liquid may escape from the average pitot tube 20 to the flow path in the pipe 1. Further, when a sudden pressure reduction occurs in the pipe 1 and then the pressure acting on the pressure impulse line 25 is abruptly reduced, a phase change may occur in the pressure impulse line 25. Thus, in order to solve the problem, an additional cooling device is required. Further, the local bidirectional flow tube 30 is problematic as follows. That is, the local bidirectional flow tube 30 is not capable of being used to measure the average flow rate of the fluid in the pipe 1.
Further, when two-phase flow occurs in the pipe 1, or the fluid suffers a phase change due to a the sudden pressure reduction in the pipe 1, or stratified flow of a two-phase fluid occurs in a horizontal pipe, the local bidirectional flow tube 30 must be displaced to another position to measure the average flow rate.